Exact measurements

| November 30, 2006

Knowing how to use accurate measuring tools will set you apart from the amateurs.

There are many things on a truck you can’t do by eye or feel. One of the biggest differences between an amateur mechanic and a competent technician is knowing how to use special tools that precisely measure important values.

Even a beginning do-it-yourself technician should know how to use a torque wrench because using one is essential to properly torque wheel lug nuts. Learning to use more complex tools will help you if you decide to become an owner-operator. Many independent truckers graduate to doing major work on their vehicles to save money by taking the time to learn proper procedures, including special tool use.

We visited Pennco Technical Institute in Blackwood, N.J., where instructors Bill Brandt and Nelson Barr showed us the details of using special tools.

Using a torque wrench
The word “torque” means twisting force. Torque is equivalent to weight when measured a certain distance from the center of a shaft, bolt or screw. “Pounds-feet,” often called “foot pounds,” are the specified number of pounds measured one foot from the center of a shaft or bolt. “Inch-pound” just means torque measured on a different scale – only one inch from the shaft’s center. For example, 120 inch-pounds would be equivalent to 10 pounds-feet.

Torque wrenches are extremely important because one of the secrets of good mechanical work is tightening something with just the right amount of force. Fasteners need to be tight enough to keep parts from bending or flexing in operation, which can cause them to loosen. But too tight is just as bad as not tight enough, because screws and bolts stretch and lose strength when tensioned past a certain point.

In a few cases, where such fasteners must be used only once, the bolt is often stretched just to the point of maximum strength. Once this has been done, it is permanently altered or “fatigued” and must be replaced when the part is removed and then reassembled. In both cases, creating exactly the right torque or twisting force is critical if the assembly is to hold together.

This inch-pound torque wrench (A) is used to check torque when assembling smaller parts. It can use various sizes of sockets with the proper extensions or adapters. You position the back pointer to zero, and then apply the torque by twisting the wrench. After you release the torque, the black pointer will remain at the point of maximum torque to record what you did.

This more traditional design (B) is for larger bolts and higher torques. When you apply torque, the shaft rotates or deflects slightly while the pointer stays straight and indicates the torque on the scale in either pounds-feet or Newton-meters.

A third type for larger bolts and torque ratings is a click type. This type wrench is used to torque cylinder heads and wheels. It is set by turning the sleeve until its edge lines up with the number of pounds-feet desired. You install the proper size socket, place the socket securely over the bolt, and turn with gradually increasing force. It’s important to make sure to hold the socket on tight with your other hand so it won’t slip off and damage the bolthead, Brandt says. The wrench will click and release slightly when reaching the rating.

After use, always make sure to turn the sleeve back until the torque is zero to unload the spring. This prevents fatigue of the spring, which would alter the calibration.

Calibration is a way to check that a torque wrench is still accurate with a known-accurate wrench. Study the instructions that come with the tool to see how often it needs to be calibrated.

Using a micrometer
Often known as a “mic,” (pronounced “mike”), a micrometer measures very short distances like clearances between close-fitting parts with great precision. Mics take some education to read because of the way they are constructed. To get the final reading, you need to read several scales and add them together. This relates to the fact that we use a decimal number system with the numbers in each position representing 1/10 the distance represented by the number to its left.

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